The present invention relates to a method and machine for rolling a workpiece formed of metal, for example steel, aluminum and the like.
More particularly, the present invention relates to such a method and machine employable in the rolling mill technology for rolling workpieces such as metal plate, shaped metal, bar metal, etc.
Further particularly, the present invention relates to such a method and machine whereby it is possible to subject the metal workpiece to compressive forces causing plastic deformation of the workpiece, but under conditions of reduced coefficient of friction and thereby reduced rolling load for a given reduction ratio.
Conventionally, both hot rolling and cold rolling are employed for reducing metal workpieces. Typically hot rolling of steel, aluminum and the like workpieces such as plate, band, bar and strip stock is achieved in a hot strip mill. On the other hand, cold rolling of such stock typically employs a tandem cold mill. In such mills the stock is passed between a pair of upper and lower rotating work rolls to achieve reduction of the stock by plastic deformation thereof under compressive force conditions. High friction is generated between the workpiece and the work rolls during these rolling operations. It is desirable to decrease such friction to reduce the load required to effect plastic deformation and also to prevent abrasive wear of the work rolls and the creation of surface flaws and defects in the workpiece being rolled.
It is known to decrease such friction by employing a lubricating agent, for example a fluid, to be applied to the surfaces of the workpiece and/or to the surfaces of the work rolls. Past attempts to reduce friction by the use of a lubricating agent have suffered from certain disadvantages.
In cold rolling, to provide the necessary reduction radio it is necessary to impart extremely high pressures to the work rolls to increase the rate of compressive force. As a result, arcuate surfaces of the rolls contact opposite surfaces of the workpiece under extremely high pressures. This prevents a lubricating agent from directly entering between the contact surfaces of the workpiece and the work rolls. As a result, the lubricating agent is only partially effective and does not in fact provide sufficient lubrication.
In the case of hot rolling, the temperature of the workpiece can reach approximately 900.degree. C. to 1200.degree. C. during a hot rolling operation. The lubricating agent is applied to the work rolls, but the work rolls also are heated due to the high temperature environment. Lubricating agents burn at a relatively low temperature, for example 200.degree. C. As a result, the lubricating agent ceases to function as a lubricant, and only a relatively slight amount of lubricating agent will remain on the work rolls. This results in insufficient lubrication of the work rolls, with a consequent abrasion of the work rolls. The poor lubricating effect and the abrasion of the work rolls increase the coefficient of friction between the workpiece and the work rolls. As a result, a large rolling load is required to maintain a desired reduction ratio of the workpiece.
In many actual conventional hot rolling operations, the only lubricating fluid actually present between the work rolls and the workpiece is air. Air cannot operate as a boundary film due to its compressibility. As a result, the entire workpiece is placed in metal-to-metal contact with the work rolls, i.e. every surface region of the workpiece is brought into a state of boundary friction with the work rolls. As a result, the coefficient of friction between the work rolls and the workpiece becomes at least 0.2 or more (.mu..gtoreq.0.2). This causes the rolling load to increase, the surfaces of the work rolls to become worn and deteriorated, damage to the workpiece, etc.
As a result of the above disadvantages of conventional rolling mills, the rolled workpiece has, in the widthwise direction thereof, surface irregularities such as local projections or high spots, a defective shape, and zig-zag surface configurations.